High-Intensity Bragg Reflection of a Femtosecond Laser via Ionized Structures in Air

Plasma optics can manipulate high power lasers at far higher fluences than solid-state components. The difference in index of refraction between air and plasma provides one method to control high intensity light. We use the spatial beat wave between 50-fs beams crossed at an angle to create a volumetric ionization grating in air; the intensity modulation produces a one-dimensional grating with alternating regions of plasma and neutral gas. We experimentally demonstrate redirection of millijoule-scale 40 fs pulses by an ionization grating and characterize the effects of spatial and temporal overlap of both pump beams and the probe. Direct interferometry measurements capture the evolution of plasma density in the grating during its sub-picosecond formation and subsequent exponential decay over tens of picoseconds. Three-dimensional simulations reproduce observed diffraction patterns and efficiencies. Control of light at intensities above the damage threshold of solid optics suggests a path towards smaller beam diameters for high power lasers.